Electrostatic spray guns are used for various applications to spray liquid and powder coatings onto various moving or stationary objects and parts. Generally, the coating is atomized and emitted as a mist from the end of the gun having a high voltage electrode. The electrode creates an electric field and an ion flux through which the sprayed particles pass, and the ion bombardment electrostatically charges the atomized coating particles passing through the ion-rich electric field. The electrostatically charged coating particles are then directed toward the object being sprayed, which is typically electrically grounded, so that the charged particles emitted from the end of the gun are attracted to the object to provide better adherence and coverage of the object with coating material. "Spray gun" as used herein includes any electrostatic spray device, whether or not hand-held, and whether or not configured in the shape of a pistol.
Many hand-held electrostatic spray guns utilize an internal high voltage power supply to charge the electrode. These spray guns have a low level voltage input, for example from 12 to 30 volts DC, which is boosted by the internal power supply of the gun to a level that is desirable for the charging electrode, usually 50 kilovolt (KV) or more. A low voltage level input allows the input power line to the gun to be smaller and more flexible, and hence more maneuverable, because it is not necessary to insulate the line to handle high voltage levels. The internal power supply has a voltage multiplier section or circuit that increases the low level supply voltage to a voltage level that is sufficiently high to electrostatically charge the spray particles. The multiplier circuit generally operates according to a characteristic power loadline which relates a) the output or load current delivered to the electrode, i.e., the amount of current, in microamperes (.mu.A), drawn to charge the spray particles, to b) the output voltage at the charging electrode.
The characteristic power loadline of a spray gun multiplier circuit determines the quantity and distribution of charge delivered to the spray particles, and thus controls the quality of the coating on the object being sprayed. Typically, the characteristic power loadline of the gun multiplier circuit is such that the output electrode voltage decreases as the load current delivered to the spray particles increases, and the external impedance between the charging electrode and ground reference decreases. The loadline determines the rate at which the output voltage drops with an increase in load current. The load current will tend to increase and the voltage on the electrode will consequently decrease as the grounded article being sprayed moves closer to the tip of the spray gun electrode, such as when objects moving along a production line pass closer to the gun electrode or when the gun (and electrode) is actually manipulated closer to the object to spray recesses or cavities located in it. Regardless of how the load conditions change, the load current and the output voltage generally will fluctuate during the spray application, affecting the quantity of charge on the particles and the quality of the spray coating. Therefore, while the gun may operate in the optimal range along the power loadline for a period of time during a spray application, at other times during the same spray application, it operates non-optimally because of fluctuating load conditions. For example, at a given load current the corresponding output voltage may be adequate for a particular spray application condition; however, should the gun move closer to the object being sprayed, increasing the load current, the reduced output voltage may no longer be adequate to properly charge the spray particles.
Generally, the input voltage level to the gun and multiplier circuit determines the operating power loadline of the spray gun. A problem with currently available spray guns is that they utilize power supplies with essentially fixed input levels and fixed operating loadlines. That is, they have loadlines which are desirable for certain load conditions during the spray application, but are inadequate for other load conditions during the application where the load conditions have changed. Therefore, for a particular spray application, a spray gun user is forced to choose a power supply multiplier circuit having a loadline which hopefully is suitable for a majority of load conditions likely encountered during the application, and to settle for non-optimal operation should the conditions change and cause the load to vary significantly from that selected.
One solution that has been proposed to rectify the problem of having a varying output voltage for different load current conditions, is to maintain the output voltage constant despite the changing load current levels. However, this is not a satisfactory solution for a least two reasons. First, the constant output voltage may not be the optimal operating voltage for a particular spray application once the load has changed. Secondly, when using high voltage electrodes and circuitry in an electrostatic spray gun, there is an inherent danger of electrical arcing at the gun nozzle. If arcing occurs in the presence of flammable spray material, ignition may result. The point at which arcing occurs is influenced by the energy delivered to the electrode, which, in turn, is dictated, by the output capacitance E=1/2CV.sup.2. Power supplies are usually designed to have a loadline that is safely below the ignition point, so that when the current increases, the voltage decreases by a predetermined amount and the resulting energy level is maintained at a safe point. However, by maintaining the output voltage constant, the available discharge energy may increase to a level that is dangerous when used with a flammable spray material.
An additional drawback of currently existing spray guns having power supplies and multiplier circuits with constant loadlines is that multiple spray gun power supplies are often necessary to handle different spray applications. For example, a power supply having a particular operating loadline may be sufficient for one spray application, but not for another application, such as, where the gun nozzle has to be moved closer to the part being sprayed to coat a recess therein. Because of this, a user with a variety of spray applications is forced to purchase multiple gun power supplies. With guns having self-contained, or internal power supplies, this can be a severe financial burden.
While varying load conditions present the problems of low coating quality and adherence, and quite possibly the hazards of arcing and ignition of the spray material, additional problems can also arise. For example, a very low load current and the resulting high electrode output voltage stress the electrical components of the spray gun power supply, and specifically, the components of the voltage multiplier stage and its associated circuitry. The voltage multiplier circuit and the associated circuitry which supplies high voltage to the charging electrode are typically surrounded with an insulating dielectric material of predetermined thickness designed to isolate the high voltage circuitry from ground potential. The insulative material, if it is not thick enough, may electrically break down and begin to conduct electricity when subject to the very high voltages that exist in the multiplier circuit. This insulation, therefore, must have a particular minimum thickness to withstand the high voltage levels in the power supply and prevent electrical breakdown of the insulation, this minimum thickness of insulation being referred to as the "isolation distance". The isolation distance is determined by the maximum voltage level that may exist in the multiplier circuit.
The maximum multiplier output voltage and the associated electrode voltage is achieved when the load current is at 0 (.mu.A) microamperes or what is considered the "no load" condition. For a particular multiplier the "no load" condition may correspond to an output voltage above 120 KV, and quite possibly above 150 KV. Therefore, the insulation surrounding the high voltage sections of the power supply must have a minimum thickness dimension or isolation distance that can withstand the maximum voltage at the "no load" point, and so, the isolation distance is determined by the "no load" voltage level. A typically reliable isolation distance requires approximately one rail (or one thousandth of an inch) of insulating material per every 400 volts that the insulation must withstand. For a "no load" output voltage level of a 150 KV, this would correspond to an isolation distance of approximately 0.375 inches. Such a large amount of insulation material around the multiplier circuit and other high voltage circuitry in the power supply makes the spray gun heavy and bulky. However, reliable performance of the power supply dictates that a minimum isolation distance must be maintained or the insulation may break down during the spray operation and render the power supply inoperable.
It is an object of the present invention to provide an improved power supply for an electrostatic spray gun which provides optimal, or near optimal, particle charging regardless of load variations encountered during operation, such as variations in the distance between the high voltage electrode and the object being coated, and which further provides for a reduction in the insulation required for the high voltage circuit components to insure safe operation under "no load" conditions.
It is further an objective of the present invention to provide a spray gun power supply which eliminates the necessity of purchasing several guns and/or high voltage supplies to handle different spray applications.
It is a still further objective of the present invention to provide a spray gun power supply that reduces the "no load" voltage so as to reduce the required isolation distance of the insulation, thereby providing a lighter, less bulky and more reliable spray gun.